Sharing composables across components in Vue - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

How does a shared composable maintain state across components?

Consider a composable that returns a ref counter. If two components use this composable, what will happen to the counter state?

Vue

import { ref } from 'vue';

export function useCounter() {
  const count = ref(0);
  function increment() {
    count.value++;
  }
  return { count, increment };
}

A Both components share the same counter state and increment together.

B Each component has its own independent counter state.

C The counter state is reset to 0 every time a component uses the composable.

D The counter state is shared only if components are nested.

💡 Hint

Think about what happens when a function returns a new ref each time it is called.

Each call to the composable function creates a new ref instance. So each component using useCounter gets its own separate state.

📝 Syntax

intermediate

Correct way to share reactive state across components using composables

Which option correctly shares a reactive count state across multiple components using a composable?

Vue

import { ref } from 'vue';

// Which composable shares the same count instance across components?

A

const count = ref(0);
export function useCount() { return { count }; }

B export function useCount() { return { count: 0 }; }

C export function useCount() { const count = ref(0); return { count }; }

D export function useCount() { const count = 0; return { count }; }

💡 Hint

Where should the ref be declared to share state?

Declaring count outside the function creates a single shared instance. Declaring inside creates a new instance per call.

🔧 Debug

advanced

Why does the shared composable state not update in the component?

A composable exports a reactive object declared outside the function. The component uses it but the UI does not update when the state changes. What is the likely cause?

Vue

import { reactive } from 'vue';

const state = reactive({ count: 0 });

export function useState() {
  return state;
}

// In component:
// const state = useState();
// state.count++;
// But UI does not reflect changes.

A The component must use <code>toRefs</code> to unwrap reactive properties for reactivity.

B The reactive object must be wrapped in a ref to update the UI.

C The component did not destructure the reactive object, so reactivity is lost.

D The reactive object is mutated outside Vue's reactivity system.

💡 Hint

Think about how Vue tracks reactivity when destructuring reactive objects.

Destructuring a reactive object breaks reactivity. Using toRefs preserves reactivity for each property.

❓ state_output

advanced

Output of shared composable with singleton pattern

Given this composable and two components using it, what is the final value of count.value after both components call increment() once?

Vue

import { ref } from 'vue';

const count = ref(0);

export function useSingletonCounter() {
  function increment() {
    count.value++;
  }
  return { count, increment };
}

// Component A calls increment()
// Component B calls increment()

A undefined

B 1

C 0

D 2

💡 Hint

Remember the shared count is a single reactive reference.

Since count is declared outside the function, both components share it. Each increment adds 1, so total is 2.

🧠 Conceptual

expert

Best practice for sharing composable state with reset capability

You want to share a reactive state across components but also allow resetting it to initial values. Which pattern is best?

A Use provide/inject to share state but do not expose reset functionality.

B Declare reactive state inside composable and recreate it on each call to reset.

C Declare reactive state outside composable and export a reset function that modifies it.

D Use a global variable and update it directly without Vue reactivity.

💡 Hint

Think about how to keep one shared reactive state and still allow controlled changes.

Declaring reactive state outside the composable shares it. Exporting a reset function lets components reset state consistently.